This invention relates to the investigation of underground medium with electromagnetic energy, and more particularly to improved radiating and receiving structures for communicating electromagnetic energy between a logging tool and a surrounding earth formation for determining the electrical permittivity and/or the electrical conductivity of the formation, and to improved logging tools based thereon.
Techniques are known for investigating underground formations traversed by a borehole by moving a sonde through the borehole and making measurements versus depth of certain physical properties of the surrounding formations to obtain logs from which it is possible to derive information useful for the exploration and/or extraction of mineral materials or fluids from the formations around the borehole.
Such measurements make use of various techniques, including electromagnetic waves, in order to determine certain parameters in accordance with the behavior of the electromagnetic waves in the formations. For instance, electrical conductivity measurements can be obtained in formations traversed by a borehole by electromagnetic induction as described, for example, in the U.S. Pat. No. 2,582,314 (issued Jan. 15, 1952 to Doll). The principle of electromagnetic induction, generally, is that a transmitting coil mounted on a sonde is energized by an oscillator operating at a suitable frequency, for example of the order of 20 kHz, to induce currents in the surrounding geological formations. The value of these currents, which flow along substantially circular lines centered on the centerline of the borehole, depends on the conductivity of the formations in which they originate. These currents produce an electromotive force in typically several receiving coils mounted on the logging sonde at given distances from the transmitting coil. An analysis of the parameters of the output signal of these receiving coils in relation to the signal transmitted makes it possible to obtain information on the conductivity of the formations traversed by these currents.
Induction conductivity measurement (induction logging) is a basic technique in the investigation of geological formations traversed by a borehole. It compliments electrical resistivity measurement methods, which are based on electrode-type tools. Induction conductivity measurement is indispensable when the medium inside the borehole (which in exploratory boring is generally filled with mud intended to stabilize the wall of the borehole) is not a good conductor of electricity and does not allow the use of electrode tools. More recently, tools for measuring certain properties of the formations around a borehole have been proposed which involve the propagation of electromagnetic energy in the formations at substantially higher frequencies than the frequencies used in induction logging. In these techniques, radio frequencies are used in a range which can extend from as low as about 1 MHz to about 1.1 GHz and beyond.
It is known that the parameters characteristic of the propagation of an electromagnetic wave in a medium such as geological formations depend both on the conductivity and dielectric constant of these formations. The attenuation of an electromagnetic wave propagating over a distance D in a medium which tends to dissipate the electromagnetic energy varies according to the expression: EQU e.sup.-jkD ( 1)
in which e is the symbol of the exponential; j is the imaginary operator; D is the distance traveled by the energy; and k is a complex propagation constant defined by the formula: EQU k.sup.2 =-j.omega..mu..sub.o (.sigma.+j.omega..epsilon.) (2)
In this equation, .omega. is the angular (radian) frequency, considered (.omega.=2.pi.f); .mu..sub.o is the magnetic permeability of the medium; .sigma. is the conductivity of the medium; and .epsilon. is the dielectric constant or electrical permittivity of the medium.
If one considers a nonconducting medium in which .sigma. equals zero, from the expression (2) we see that the constant k is a real term. The exponent of the exponential of expression (1) is then a pure imaginary term which corresponds only to a phase shift in the expression of the attenuation of the transmitted signals. In other words, the propagation of the waves in this medium takes place with geometrical amplitude attenuation and without overall energy attenuation. As the conductivity of the medium increases (conducting drilling mud, for example), the term .sigma. becomes much higher than the term j.omega..epsilon.. According to expression (2), the term k.sup.2 tends to become purely imaginary. The exponent of the exponential of expression (1) then becomes a term having an imaginary component and a real component substantially equal to each other. As the propagation constant k continues to increase with conductivity, the real component of the attenuation increases exponentially with k. Thus, as a first approximation the phase shift increases with the electrical permittivity while the amplitude attenuation increases with conductivity.
In order to measure these characteristic parameters, generally at least two receivers are spaced longitudinally with respect to each other and a transmitter. The distance from the transmitter to the nearest of these receivers longitudinally in the direction of the borehole determines the depth of the formation that can be reached for the measurement. The distance between the receivers determines the thickness of the formation over which the measurements of the propagation characteristics of the transmitted wave are obtained. These characteristics are, notably, the relative attenuation of the signals picked up by the near receiver and the far receiver, and the phase shift between the signals received by the near receiver and the far receiver.
The influence of the conductivity of the formation on the attenuation and phase shift becomes predominant as the investigation frequency drops. As the frequency increases into the microwave region, the influence of the electrical permittivity of the formation becomes predominant.
To obtain measurements of one or the other of these characteristic parameters (the electrical conductivity and the electrical permittivity), two simultaneous or consecutive measurements must be made of the propagation of electromagnetic waves for each formation zone of interest, for example one relative attenuation measurement and one relative phase measurement. U.S. Pat. No. 4,052,662 (issued Oct. 4, 1977 to Rau) discloses a tool operating within the microwave frequency range for determining the propagation characteristics of electromagnetic waves in a medium near the wall of the borehole. This tool includes a sonde equipped with pads designed to be applied against the wall of the borehole. On this pad are mounted a transmitting antenna and several receiving antennas of the cavity backed slot type. At an operating frequency of 1.1 GHz, the attenuation and the phase shift of waves picked up by the receiving antennas are measured to obtain the value of the dielectric constant of a zone of small thickness around the borehole immediately beyond the mudcake. At such high frequencies, the value of the dielectric constant of the investigated medium has a decisive influence on the attenuation and phase shift measurements. The influence of the conductivity of the investigated medium on these measurements becomes increasingly smaller as the frequency rises. The combination of attenuation and phase shift measurements makes it possible to completely eliminate the influence of the conductivity to determine the electrical permittivity of the investigated medium.
To obtain greater depth of investigation, it is necessary to space the transmitter and the receivers at distances which render pad mounting difficult to implement. It is then preferable to install the receivers and the transmitter directly on the mandrel of the logging sonde. Since the distance to be traveled by the electromagnetic waves increases with the investigation depth and the attenuation of an electromagnetic wave in a medium in which it propagates is an increasing function of frequency, one is then led to use lower operating frequencies, for example 20 to 30 MHz.
Electromagnetic logging tools are thus known which are equipped with a transmitter and receivers mounted on a mandrel at distances which can be of the order of a meter or more to obtain measurements covering zones located at a radial distance greater than a meter with respect to the borehole centerline. Such a tool is described in U.S. Pat. No. 4,185,238 (issued Jan. 22, 1980 to Huchital and Tabanou). A transmitter at the bottom of the mandrel operates at a given frequency. The midpoint of a first pair of longitudinally spaced receivers is at a first distance from the transmitter to obtain a relative attenuation measurement of the signals coming from the transmitter through the surrounding medium. The midpoint of a second pair of longitudinally spaced receivers is at a second distance from the transmitter, greater than the first distance, to obtain a measurement of phase shift or relative phase between the signals reaching them. The first and second distances are selected so that the attenuation and phase shift measurements performed by the first and second pairs of receivers respectively pertain to the same depth of investigation in the formation. It was determined that the measurements of the attenuation and relative phase of waves propagating through formations were affected in a different manner by the distance between the zone of interest and the borehole centerline. Thus, to obtain measurements of the phase shift caused by the propagation of waves in a formation zone at a given distance from the borehole, it was necessary to use a pair of receivers located at a greater distance from the transmitter than the distance between the pair of receivers used for wave attenuation measurements in this same zone.
In general, the radiation transducers (transmitters or receivers) used for electromagnetic logging, whether pad or mandrel mounted, must meet certain conditions. In particular, they must be adapted to the transmission of energy in highly dissipative media, i.e. where it is accompanied by considerable losses. During transmission these transducers must thus be capable of transmitting large amounts of energy to the surrounding medium, while during reception they must be capable of picking up signals of extremely low level. Moreover, these transducers must have particular directivity characteristics. In general, in electromagnetic logging techniques, one seeks to favor the propagation of waves in the direction of the formations rather than the propagation of these waves longitudinally in the borehole. It is thus important to ensure that the transducers used for this purpose have well determined directivity characteristics.
In one known technique, disclosed in United Kingdom Pat. No. 1,088,824 (Shell Internationale Research Maatschappij N.V.), two electrodes together with the rock formation and borehole fluid form a capacitor. In the frequency range 150 MHz through 1500 MHz, the electrodes preferably constitute a dipole aerial. According to another prior art technique (see, for example, the Inventors Certificate of the USSR in the name of Daev, No. 177,558), the transmitters and receivers used for the transmission of electromagnetic waves between the tool and the surrounding medium are toroidal coils whose centerline is directed along the centerline of the drilling tool mandrel. Other types of coils for the transmission and reception of electromagnetic waves are disclosed in U.S. Pat. No. 3,891,916 (issued June 24, 1975 to Meador et al.) and the aforementioned Huchital and Tabanou Patent. These coils operate as dipoles, which have good directivity.
The use of coils makes it possible to remedy to a certain extent the disadvantages associated with antennas forming capacitor plates, such as disclosed in the aforementioned UK Pat. No. 1,088,824, which would be shorted by a conductive borehole fluid. In particular, coils are capable of operating in slightly conductive drilling fluids. This improvement has limits, however, and the level of the signals which reach the receivers after propagation through the formations being investigated often is extremely low. Considerable precautions are necessary to avoid the deterioration of the signals picked up by the receivers for subsequent processing. It is also necessary to provide extremely sensitive electronic measurement circuits which make the construction of the tool more difficult and its operation more complex. This is particularly the case for the circuits associated with coils relatively far from the transmitter, such as for example the phase shift measuring circuits disclosed in the aforementioned Huchital and Tabanou Patent.
Moreover, the efficiency of traditional coils is extremely low due a number of factors. A traditional coil can be thought of as comprising, equivalently, an inductance coil connected in series with a resistor. This coil is supplied with electromagnetic energy by an oscillator 86 through a coaxial cable. The traditional coil is very highly reactive due to the high value of the equivalent inductance coil. Since the coaxial cable supplying this coil is designed to give it an essentially active energy to be radiated, an impedance matching defect occurs which is in itself the cause of poor energy transmission efficiency at very high frequency. The impedance matching defect results in the establishment of a system of standing waves between the oscillator and the coil, and the maintenance of these standing waves consumes a very large fraction of the energy which transits through the coaxial cable. Under these conditions, the efficiency of the transmission of energy between the oscillator and the coil hardly exceeds 10%. Tuning circuits are necessary to place the coil in a resonant condition, but these tuning circuits are difficult to manufacture, are subject to radiation leakage, and require a significant amount of space in the logging tool. Furthermore, considerable ohmic losses occur due to the windings of the traditional coil. Dielectric losses also result from the capacitive link between the conductors of the coil proper and other parts of the logging tool that are grounded. In all, the power radiating by a coil type radiating system is hardly more than about 1% of the power available at the output of the supply oscillator. The same phenomenon holds for a receiving coil.
Another disadvantage of known coils arises from the fact that the attenuation of an electromagnetic wave propagating in a medium increases greatly with the electrical conductivity of this medium. Thus, with known tools, when the resistivity of the drilling mud is lower than 0.1 ohm per meter, the attenuation of the electromagnetic waves in the drilling mud does not allow utilizable information to be obtained from the propagation of electromagnetic waves through the formations being investigated. By increasing the diameter of the coils, however, the power radiated by the coils could be increased and the drilling fluid thickness traversed by the electromagnetic energy (and hence the attenuation) could be reduced. However, the outer diameter of the tool is limited by the size of the boreholes in which the tool is to be used and by considerations relative to overall dimensions.
Such known coils have other disadvantages as well. They are difficult to manufacture, assemble, and mount on the logging tool, which increases costs. In particular, variation in the spacing of the coils along the mandrel due to thermal expansion in the borehole is difficult to minimize or take into account.
The poor efficiency of known coils has other implications as well. Electromagnetic propagation tools also include electronics for processing the signals picked up by the receiving antennas which, owing in particular to the low level of these signals, must be located near these antennas. The electronics is normally housed either in the upper part of the sonde body or in a case specially attached to this sonde body, to which the wireline cable is coupled. The transmitter must thus be located under the receiver toward the bottom part of the tool 30, and its power oscillator must be placed nearby in order to limit the length of the connecting coaxial line. This oscillator is powered via electrical conductors coming from the surface through the wireline cable conductors which pass through the sonde body and hence pass near the receiving antennas. In view of the low level of the signals received, it is necessary to isolate the receiver circuits from the disturbing influence of the currents carried by the power supply conductors of the oscillator 60.
A solution applied in certain cases is to supply the oscillator through a battery during the measurement periods when the transmitter sends radiation in the direction of the formations, this battery being rechargeable outside of the operating periods of the electromagnetic detection system. However, this solution calls for the use of a battery in the hostile environment in which logging tools are required to operate, resulting in particular to exposure to very high temperatures. The attendant disadvantage are fragility, an insufficient energy reserve for long-duration logging operations, and poor reliability. An alternate solution is to surround the conductors which supply the oscillator from the suspension cable of the tool with a screen made up of a longitudinal metal tube that traverses the sonde up to the oscillator. In the case of coil-type tools where the transmitted or received power depends on the flux traversing the coils, the use of the tube is at the expense of the surface area available for the flux.
Limitations in detection and processing electronics, in combination with known coil antennas, has other implications as well. Improvement in the resolution of the investigation while maintaining a desired investigation depth is sometimes desirable; however, known electromagnetic logging tools are limited in resolution for a given depth of investigation. Depth of investigation depends on the distance between the transmitting antenna and a receiver antenna pair, while resolution depends on the distance between the receiving antennas of the pair. As taught in the aforementioned Huchital and Tabanou Patent, each receiving antenna pair performs a differential measurement of the variation in certain propagation parameters such as the attenuation or the phase shift produced by a formation zone whose thickness is defined by the spacing of these receivers. Although decreasing receiving antenna spacing in principle improves resolution of the measurement, the spacing of the receiving antennas of the pair must be of sufficiently magnitude to permit the detection and measurement of the values of the parameters of the received waves by the detection and processing electronics. Thus, for example, if one wishes to detect the differences in phase shift to within a fraction of a degree, the spacing of the two corresponding receivers must be sufficient so that the variations in this parameter between two formations of a different nature to be distinguished exhibit at least this value over the considered formation thickness. The same concern applies to attenuation measurement. This limitation in resolution for a given depth of investigation is a disadvantage of known electromagnetic logging tools.
It is also known that induction measurements are particularly suitable for certain borehole domains and that resistivity measurements are particularly suitable for certain other borehole domains. Systems combining induction and resistivity measurement systems are known, but a disadvantage of combining the two measurement systems arises from the sensitivity of the antenna coils to the distorting effect of the conductive electrodes in proximity therewith. One approach to overcoming this disadvantage is addressed in U.S. Pat. No. 3,124,742 (issued Mar. 10, 1964 to Schneider), which discloses an electrode system having a number of individual electrodes, each comprising a closed loop formed by a conductor of relatively small cross-sectional area and a series of plates that form a discontinuous electrode encircling the longitudinal axis. Such a system disadvantageously precludes the use of relatively massive electrodes having better investigation depth performance.
It is also known that relatively shallow resistivity measurements are available with the use of pad-type logging tools; see, for example, U.S. Pat. No. 2,712,630 (issued July 5, 1955 to Doll). Furthermore, it is known to use relatively shallow resistivity measurements to determine the dip angle and the azimuthal angle of formation bedding planes by passing through a borehole a "dipmeter" tool having a plurality of circumferentially-spaced pad-mounted electrodes. Although these conventional multiple-pad dipmeter devices provide generally satisfactory results, an inherent difficulty is the necessity to insure that the pads make reasonably good contact with the surrounding formations when the borehole fluid is relatively nonconductive (e.g., oil based drilling mud). Another type of dipmeter device, the so-called "induction dipmeter," has been proposed to overcome this disadvantage. The induction dipmeter, which is based on the principles of induction logging to measure dip angle, includes conventional induction coils wound on an insulating mandrel or on pads urged against the borehole wall. The disadvantages associated with coils affect such induction dipmeter devices as well.
In the field of atmospheric and space telecommunications, a known technique is to use antennas comprising a dielectric plate, one side of which has printed thereon an elongated conducting element while the other side is metalized to form a second conducting element or ground plane. These antennas, known as bi-plate lines, used printed circuit manufacturing techniques. The efficiency of these antennas, however, is not as good as can be achieved with conventional above-ground antennas, which are dimensioned according to the propagation wave-length of the radiation they are designed to transmit in air or vacuum. Nonetheless, they have been found to be well suited to omnidirectional type transmission in air or vacuum in atmospheric and space telecommunications applications because these applications use frequencies of several hundred megahertz where the efficiency of antennas of this type, which increases as the square of their utilization frequency, is acceptable. Furthermore, because the propagation of electromagnetic waves in these media takes place practically without loss, the relatively lesser efficiency of bi-plate antennas in these applications is compensated by their advantages. These advantages include in particular the relative easy with which the bi-plate antenna can be shaped in a relatively small volume to the form of atmospheric or space vehicles.